Priority is claimed to Korean Patent Application No. 10-2004-0003253, filed on Jan. 16, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a standard sample for transmission electron microscope (TEM) elemental mapping and TEM elemental mapping using the same, and more particularly, to a standard sample for transmission electron microscope (TEM) elemental mapping which can be used to correct results of elemental mapping of nanometer-sized thin films, which are obtained by energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) methods using TEM, and to optimize mapping conditions, and TEM elemental mapping method using the same.
2. Description of the Related Art
Recently, industrialized countries have strategically intensified ISO international standardization activities to widen the world's industrial market. In particular, public research centers where analyzing equipment is operated, government-sponsored research centers, corporate research centers, analyzing service centers, and the like have contributed to the formation of a large-scale analysis marketplace. In this market, a microbeam analysis technique plays a pivotal role in R&D and industrialization of nanometer scale techniques. However, the microbeam analysis technique has not been standardized, and also the manufacture, research, and development of standard samples for microbeam analysis are far behind a desirable level.
An elemental mapping method is an imaging technique in which 2-dimentional images of the layer structure and the elemental distribution of multi-layered thin films are produced. Transmission electron microscope (TEM) mapping methods, which are used to obtain elemental mapping images of multi-layered thin films, can be divided into energy dispersive spectroscopy (EDS) mapping using a characteristic X ray and electron energy loss spectroscopy (EELS) mapping. Since thicknesses of multi-layered thin films included in devices are on a micro-scale, the resolution of an elemental mapping device must be on a nanometer scale. Accordingly, a standard sample is needed to correct elemental mapping images and to optimize mapping conditions.
In TEM elemental mapping of nanometer sized thin films, the thickness, chemical composition, and distribution of a standard sample must be precisely known. As a result, the standard sample must have a structure, which can be easily analyzed to obtain the thickness, elements, and the structure thereof.
In order to analyze a multi-layered thin film sample having a thickness of 1–2 nm, X-ray reflectivity (XRR) and TEM, rather than scanning electron microscopy (SEM) or ellipsometry, are preferred to efficiently obtain precise analysis results. If TEM is used to analyze the elements and the structure of a nanometer-sized thin film having the thickness of 1–2 nm, TEM/EDS using a field emission electron gun and TEM/EELS are used.
A conventional TEM elemental mapping method used on a nanometer-sized thin film includes identifying an element of which the nanometer-sized thin film is composed using EDS and EELS, selecting a peak energy corresponding to the element, and performing elemental mapping under a condition in which the signal to noise ratio (S/N) is good. Although a mapping image formed from the nanometer-sized thin film using this method has a nanometer-sized thin film thickness difference of about 1–5 nm depending on mapping methods and conditions, the mapping thickness difference has been ignored because it is relatively small.
In principle, the use of the EDS mapping method results in a decrease in spatial resolution power of a mapping image since a characteristic X ray, which is generated when an impinging beam diffuses in a sample, is used. In addition, the spatial resolution of a mapping image can be changed by altering an acceleration voltage, a probe size, sample drift, and a sample thickness. The EELS mapping method can be used to realize an elemental mapping image of nanometer-sized thin films by high spatial resolving power because a sub-nanometer scale probe size can be obtained by FE-TEM. However, a SNR varies according to a sample thickness and an element being mapped. Additionally, depending on EELS mapping conditions, energy drift, sample drift, a probe size, and delocalization have various influences on the spatial resolution of the mapping image. These variables result in a mapping thickness difference between a measured thickness and a real thickness.
U.S. Pat. No. 6,231,668 discloses an image correction specimen for a scanning electron microscope (SEM), a scanning transmission electron microscope (STEM), and a scanning probe microscope (SPM). The image correction sample is prepared by sequentially depositing crystalline or amorphous heterolayers with different compositions. The thickness of the individual layers are less than 25 nanometers. U.S. Pat. No. 6,420,703 discloses a SEM correction standard sample in which straight metal lines having a uniform line width of less than 20 nm and a length of several tens of μm are formed using a focused ion beam technique.